[JohninCR]

Yes JohnK, thank you.  Now how do we factor the edge along the floor into the equation for the baffle?  It has an effect on the differential too, not just the boundary gain, right?

Also, I love when my instincts agree with the math.  I was actually thinking about 1/3 the benefit, but didn't want to go that far out on a limb on gut feel alone.  I guess my brain is doing the trig subconciously. LOL

[FlorianO]

At low frequency any shaped baffle can be shown to be equivalent to a circualr baffle with driver centered of some radiushttp://www.musicanddesign.com/Equivalent_Baffles.html. For a 24" x 24" baffle with centered driver at 12" x 12" the equivalent circualr baffle has a diameter of about 27". Extending the baffle to 48" tall, but keeping the driver at 12" x 12" yields an equivalent diameter of 35'". Placing either baffle near a floor will result in a 4Pi to 2Pi transition which will boost the low frequency response by about 6dB below nominally 100 Hz. The 6dB "floor" boost for the smaller baffle at low frequency is greater than the gain from the larger baffle in free space as the larger baffle only yields a 2.25dB  boost over the smaller baffle in free space.

John, I am a bit lost....

While I intuitively understand your argument, some things don't match in my mind.

Apparently in the paper quoted above putting the baffle flush with the floor the equivalent effective diameter increases by sqrt(2) i.e. Fequal decreases by half-octave.

OTOH you are talking about the boost provided by a baffle flushed to the floor ( +6 dB) vs a _longer_  baffle and an equivalent circular baffle upscaled  by sqrt(2)....

Are we talking apples and oranges or is there smth I'm missing ?

Regards,

Florian

[JohninCR]

An across the board 41% increase in D for the floor sure doesn't feel right.  That relationship is too shape and driver position on the baffle dependant.  When I visualize what's how the floor affects everything, I look at it from 2 aspects with the primary one being how high is the driver from the floor for boundary loading.  The other is how big a piece of pie does the floor represent if you imagine a circle around the driver that intersects with the floor at the baffle corners.  The bigger the piece of pie in relation to the whole, the greater the effect.

[johnk...]

Yes, Johnin CR is on the correct path. What I was trying to show was that in free space the larger baffle only has a 2.25dB effect on the SPL level at low frequency and about a 30% upward shift in Fequal. But placing the smaller baffle near a floor will have two effects: 1) will be the blockage and 2) with be the 4Pi to 2Pi transition. At low frequency the 4Pi -2Pi transition will be the dominant factor. The blockage effect will depend, as JohninCR said, on the baffle aspect ratio and just how much of the perimeter is blocked. Obviously for a narrow, wide baffle there will be little blockage effect. The 4Pi to 2Pi transition will also depend on baffle size and source location on the baffle. But when the wave length exceeds about 3 or 4 times the baffle max dimension I would expect you would be getting deeper into the transition.

It's pretty easy to extend a  dipole baffle simulation to do this. I may do it at some time. Maybe Martin Kings open baffle sheet already does it?

[MJK]

Maybe Martin Kings open baffle sheet already does it?

Yes, the worksheets take the floor boundary condition into account for both the driver and the baffle response.

[FlorianO]

An across the board 41% increase in D for the floor sure doesn't feel right.  That relationship is too shape and driver position on the baffle dependant. 

Well, in all honesty the above figure was quoted in the context of a bafffle that was 168 cm (half-wave at 100Hz) wide and its width "reduced" to 119 cm for same Fequal. I was under the assumption that scaling by that factor was linear with different widths.

thanks

[scorpion]

If I could wish, I would like the MJK-spreadsheet to be able to handle n-corner baffles ? Or if that is impossible at least 4 corners which I can define myself,
that would be great. I think that would cover almost all practical baffles.

/Erling

[MJK]

at least 4 corners which I can define myself

That would seem like a logical next step in the continual upgrade path. I am working on something else right now but that will be high on my priority list when I get back to working on the MathCad worksheets.

[MJK]

Erling wrote :

If I could wish, I would like the MJK-spreadsheet to be able to handle n-corner baffles ? Or if that is impossible at least 4 corners which I can define myself, that would be great. I think that would cover almost all practical baffles.

I just uploaded new OB worksheets that are four sided, floor standing, but variable shaped. So a trapezoidal baffle model is now possible. These new worksheets also include some new edge source math which does a better job compared to the previous versions. This is a first step and I am looking at n-cornered geometry definitions and a couple of other options.

Martin

[ttan98]

Martin,

Where? can't find it. What is the title?

ttan

[MJK]

Hi Tean Tan,

The worksheets have the same name as the older ones but the revision dates are 6/13/07. The updated feature can be found in the detailed input section, you now provide corner locations for the baffle so the four sided shape can be varied.

Hope that helps,

Martin

[ttan98]

Martin,

Applies to all open baffle worksheets(5) or just the last 3?

ttan

[ttan98]

last 3 only...I just downloaded them.

ttan